1. Field of the Invention
The invention is in the field of voltage surge protection devices, such as are used to protect telephone station apparatus from external voltage surges (e.g., lightning strikes and induction or accidental contact between telephone lines and power lines).
2. Description of the Prior Art
In transmission systems with large stretches of outdoor wiring, it is common to protect terminal equipment from voltage surges (e.g., lightning strikes) by the inclusion of a protective device between the line and ground at each terminal. Such devices should be capable of sustaining repeated voltage surges without failing but when they fail, they should fail to an electrically short circuit condition in order to safeguard the terminal equipment. A widely used class of surge protective devices includes two carbon block electrodes with parallel faces defining an air gap of the order of 50 micrometers. This is an extremely inexpensive device, however, the labor cost of replacing failed devices in the field is high. Thus, efforts have been made to extend the service life of such devices.
One such modification, sometimes known as the "gas tube" protector, consists of metal electrodes hermetically sealed in an inert gas atmosphere. Such devices typically include a carbon coating on the electrodes which tends, among other things, to increase the electron emissivity of the surface, thus facilitating the formation of the plasma discharge. One form of such a device utilizes a relatively wide gap (e.g., 500 micrometers) between parallel faces and reduced gas pressure, in order to maintain approximately the same breakdown voltage as the air gap device (U.S. Pat. No. 3,454,811, issued July 8, 1969). This wider gap spacing increases service life, since the chance of shorting failure cross the wider gap is greatly reduced. However, when the hermetic seal on such a device fails, the breakdown voltage increases to far above the safe limit. This is known as a "fail open" condition and represents a finite hazard to the terminal equipment and the user. In another group of such devices the inert gas pressure is maintained at approximately atmospheric pressure. However, this requires the use of a narrow gap (e.g., 25-75 micrometers) for a breakdown voltage within the desired safe range. This device represents an improvement over the narrow gap carbon block device because of the materials used and the inert atmosphere. It also maintains the fail-safe feature of the carbon block device, in that seal failure does not increase breakdown voltage above the acceptable level. Hence the dominant failure mode of this device is still shorting across the gap due to electrode damage.